The visual system of the giant barnacle consists of 10 photoreceptor neurons divided among three eyes, one median and two lateral, that mediate a shadow reflex withdrawal behavior of the animal. This preparation is a relatively simple one in which to study the processing of visual information by single, identified cells. We have previously described second- and third-order cells of the median visual pathway and their roles in inverting and amplifying signals from the receptors. We will test the hypothesis, formulated on the basis of preliminary evidence, that acetylcholine is the transmitter released by the photoreceptors. We will determine whether lateral receptors contact median receptors. We find that median receptors are sensitive to acetylcholine, and in the EM we can see synapses made upon them. We plan to study higher order cells of the lateral visual pathway to attempt to account for a directional sensitivity of the visual pathway. We also will continue to study the complex physiology of the I-cell. We are particularly interested in the relation between receptor voltage and I-cell voltage in adapting lights, in whether we can see quantal events due to release from the receptor, and in characterizing the synaptic input to this cell that will eventually help us explain its response to the offset or dimming of light.